Sorting the sequential arrangement of radiation treatment beams regarding well-distributed unobstructed lines of sight

ABSTRACT

Disclosed is a method for determining a positional pattern of an irradiation unit for irradiating a patient with treatment radiation. Optimal order data describing an order of the irradiation unit positions for which the statistical value is optimal is determined. The optimal order data is determined based on irradiation unit position data describing irradiation unit positions of the irradiation unit for which the imaging device has a free viewing direction onto the position of the patient, position orders data describing all possible orders of the irradiation unit positions for which the imaging device has a free viewing direction onto the position of the patient, and intersection angle data describing a statistical quantity of the intersection angles between free viewing directions of the imaging unit for irradiation unit positions which are immediately subsequent in the order described by the position orders data.

RELATED APPLICATION DATA

This application is a national phase application of InternationalApplication No. PCT/EP2017/052940 filed Feb. 10, 2017 and claimspriority from PCT Application No. PCT/EP2016/052840 filed Feb. 10, 2016both are published in the English language.

BACKGROUND

The present invention relates to a computer-implemented method fordetermining a positional pattern of an irradiation unit for irradiatinga patient with treatment radiation, a corresponding computer program, anon-transitory program storage medium storing such a program and acomputer for executing the program, as well as a system for determininga positional pattern of an irradiation unit for irradiating a patientwith treatment radiation, the system comprising an electronic datastorage device and such a computer. The invention also relates to aradiation treatment system comprising: an irradiation treatment devicecomprising an irradiation unit and an imaging device and theaforementioned computer.

So far the treatment beams defined in a radiotherapy/radiosurgerytreatment plan are processed in an order guaranteeing short travels forthe gantry of the irradiation unit, and thereby maybe guaranteeingshorter treatment times. However, many camera systems suffer from thegantry obstructing one of the camera's lines of sight depending on thegantry angle. Thus, driving the irradiation unit of a treatment deviceto the positions for emitting the treatment beams in a time-optimisedmanner may lead to undesirable obstruction of the beam path (the line ofsight between the treatment device and the position of the patient)specifically by constituents of the treatment device (such as thegantry).

The present invention allows for determining a sequence of positions ofthe irradiation unit which avoids such an obstruction.

Aspects of the present invention, examples and exemplary steps and theirembodiments are disclosed in the following. Different exemplary featuresof the invention can be combined in accordance with the inventionwherever technically expedient and feasible.

SUMMARY

In the following, a short description of the specific features of thepresent invention is given which shall not be understood to limit theinvention only to the features or a combination of the featuresdescribed in this section.

The disclosed method encompasses determining an optimal order ofpositions of the gantry of a radiotherapy/radiosurgery device so thatfor positions which are adjacent in that order images can be taken ofthe patient's body which allow for an improved reconstruction of stereoimages or pseudo-stereo images of the patient's body which are used forassessing whether the patient is placed relative to the gantry asdesired (and/or as prescribed by a treatment plan). The optimal order isdetermined such that the stereo-imaging field of view achievable forgantry positions neighbouring each other in the optimised order ismaximised e.g. on average.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described with reference to theappended figures which represent a specific embodiment of the invention.The scope of the invention is however not limited to the specificfeatures disclosed in the context of the figures, wherein:

FIGS. 1a to 1d show a prior art method of controlling the gantry of theirradiation unit for the case of a stereo camera as an imaging device;

FIGS. 1e to 1h show application of the disclosed method to the case ofFIGS. 1a to 1 d;

FIGS. 1i and 1j show the resulting intersection (reconstruction) anglesfor the irradiation unit positions of FIGS. 1a to 1h and 1e to 1 h,respectively;

FIGS. 2a to 2d show a prior art method of controlling the gantry of theirradiation unit for the case of an imaging device comprising only oneimaging unit;

FIGS. 2e to 2l show application of the disclosed method to the case ofFIGS. 2a to 2 d;

FIGS. 2m to 2o show the resulting intersection (reconstruction) anglesfor the irradiation unit positions of FIGS. 2a to 2d and 2e to 2h and 2ito 2 l, respectively;

FIGS. 3a to 3d show the combined field of view attributable tosubsequent gantry positions for the case of using a thermal camera asthe imaging device in an unordered sequence of gantry positions;

FIGS. 3e to 3h show the combined field of view attributable tosubsequent gantry positions for the case of using a thermal camera asthe imaging device in an optimized sequence of gantry positions;

FIGS. 3i and 3j show the combined field of view associated with theirradiation unit positions of FIGS. 3a to 3d and 3e to 3h ,respectively;

FIG. 4 shows application information derivable from fusion of images insix degrees of freedom, the images having been taken by the imagingdevice; and

FIG. 5 illustrates a flow diagram representing basic steps of thedisclosed method.

Throughout the figures, dashed lines indicating viewing directions (e.g.viewing directions 6, 7) indicate an obscured viewing direction, andsolid lines indicating viewing directions (e.g. viewing directions 6, 7)indicate a free (non-obscured) viewing direction. The same referencesigns have the same meaning throughout the figures.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In this section, a description of the general features of the presentinvention is given for example by referring to possible embodiments ofthe invention.

The disclosed method provides, in a first aspect, a computer-implementedmethod for determining a positional pattern of an irradiation unit forirradiating a patient with treatment radiation, the irradiation unitbeing part of an irradiation treatment device having an imaging devicefor generating a medical image of the patient. The method comprisesexecuting, on at least one processor of at least one computer, thefollowing exemplary steps which are executed by the at least oneprocessor.

In a (for example first) exemplary step, irradiation unit position datais acquired which describes (for example, defines) irradiation unitpositions of the irradiation unit for which the imaging device has afree viewing direction onto the position of the patient. The freeviewing direction is a free line of sight from the imaging device to theposition of the patient. Specifically, the free line of sight is notimpeded by a part of the irradiation unit (such as the gantry of theirradiation unit which constitutes an emitting end at which thetreatment beam is emitted from the irradiating unit). The imaging devicemay be an x-ray imaging device. The imaging device can be at least oneof an imaging device having (for example exactly) two imaging units suchas a stereo imaging device (for example, an infrared-sensitive imagingdevice like a stereo thermal camera such as a stereo infrared-sensitivecamera, or a stereo x-ray camera, specifically an x-ray imaging devicehaving two imaging units such as each a flat panel x-rayemitter/detector combination which in one example has a fixed positionrelative to the patient during the imaging process—and in anotherexample may have a variable position relative to the patient during theimaging process, a range camera or camera for measuring distance on thebasis of detecting reflections of structured light from the surface ofthe patient's body) and an imaging device having (for example only) asingle imaging unit (for example, an infrared-sensitive imaging devicelike a mono thermal camera or an x-ray imaging device having only asingle imaging unit, such as a single x-ray emitter/detector combinationwith a variable position relative to the patient during the imagingprocess). In one example embodiment, the imaging device comprises atleast two imaging units (for example, two flat panel x-rayemitter/detector combinations which constitute a stereo x-ray camera)and the irradiation unit positions are positions of the irradiation unitfor which the only one or all of the at least two imaging units have afree viewing direction.

In one example embodiment of this step, the irradiation unit positiondata is predetermined and has been generated on the basis of treatmentplan data which describes (for example, defines) an irradiationtreatment plan. The irradiation treatment plan defines for exampletreatment positions which are relative positions between the irradiationtreatment device and a treatment body part of the patient at which thetreatment radiation shall be emitted. The treatment data is in oneexample predetermined (i.e. has already been determined, specificallydetermining (e.g. calculating) the irradiation treatment plan is notpart of the disclosed method.

In another example embodiment of this step, irradiation unit positiondata is predetermined and has been generated on the basis of devicegeometry data describing an operational geometry of the irradiationtreatment device, for example the relative position between theirradiation unit and the viewing direction. The irradiation unitposition data may therefore be defined by the irradiation treatmentdevice

In a (for example second) exemplary step, position orders data isdetermined which describes (for example, defines) all possible orders ofthe irradiation unit positions for which the imaging device has a freeviewing direction onto the position of the patient. The position ordersdata is determined based on the irradiation unit position data.Determining the position orders data encompasses calculating allpossible sequences of irradiation unit positions which are associatedwith a free viewing direction of the imaging device onto the position ofthe patient. The order or sequence, respectively, of the irradiationunit positions is an order in time and space and defines an order inwhich the irradiation unit shall attain the irradiation unit positons.The order is discrete in the sense that it contains only discreteindividual positions which need not necessarily be adjacent(neighbouring) in space. Any wording herein referring to adjacent and/orsubsequent and/or neighbouring irradiation unit positions means thatthey are adjacent and/or subsequent and/or neighbouring only with regardto their location in the order/sequence of irradiation unit positionsand not necessarily in real space.

In a (for example third) exemplary step, intersection angle data isdetermined which describes (for example, defines) a statistical quantity(for example, a sum, the average or an average field of view) of theintersection angles between free viewing directions of the imaging unitfor irradiation unit positions which are immediately subsequent (i.e.neighbouring) in the order described by the position orders data. Theintersection angle data is determined based on the irradiation unitposition data and the position orders data. For example, theintersection angle data is determined per order (i.e. per orderedsequence), for example for each order, defined by the position ordersdata. The intersection angles between viewing directions define a fieldof view of the imaging device between the pairwise subsequentirradiation unit positions. For example, the intersection angles definethe extent (i.e. an outer boundary in a two-dimensional projection) ofthe field of view of the imaging device between the pairwise subsequentirradiation unit positions. In example embodiments of this step, thestatistical quantity may be at least one of a sum of the intersectionangles, the average of the intersection angles, or the average field ofview of the imaging unit between the viewing directions associated withthe intersection angles. The intersection angles in one exampleembodiment of this step consist (for example exclusively) of obtuseangles and right angles or of (for example exclusively) acute angles andright angles.

In a (for example fourth) exemplary step, optimal order data isdetermined which describes (for example, defines) an order of theirradiation unit positions for which the statistical value is optimal.The optimal order data is determined based on the intersection angledata and the position orders data. The optimal order is an order forwhich the statistical quantity is a maximum (specifically, attains amaximum value). The optimal order can then be stored for later use as anorder of positions which the irradiation unit is controlled to attainfor example for conducting radiotherapy on the patient. If more than oneorder of the irradiation unit position is associated with an optimum ofthe statistical quantity, the order of the irradiation unit positionsassociated with the smallest standard deviation is determined as theoptimal order. If more than one order of the irradiation unit positionis associated with a smallest standard deviation, the order of theirradiation unit positions associated with the smallest amount of timefor moving the irradiation unit through the order of irradiation unitpositions is determined as the optimal order. The optimisation of theorder of the irradiation unit positions is effected by finding thosepositions for which the viewing direction is not obscured (for the caseof an imaging device having only a single imaging unit, this step isoptional, as will be clear to the skilled person e.g. from FIG. 2 whichshows that no irradiation unit position is obscured in that case), andthen putting them in such an order that e.g. on average the intersectionangle between non-obscured viewing directions (free lines of sight)associated with adjacent/subsequent irradiation unit positions ismaximized.

The maximisation may additionally or alternatively be achieved byapplying an optimisation algorithm such as the one disclosed in thefollowing to the sequence of irradiation unit positions in order tooptimise the statistical measure (e.g. maximising the averageintersection angle).

For each permutation of irradiation unit positions, it is determinedwhether it constitutes an acceptable order of the irradiation unitpositions according to a predetermined (predefined) acceptancecriterion. If an order is determined to be acceptable, the optimisationprocedure ends and analysis of further permutations can be avoided,whereby the computational efficiency of the optimisation procedure isincreased compared to a brute force approach which would include allpossible permutations are analysed as to their suitability. Theacceptance criterion is in one example of this approach defined as acombination of the average intersection angle of viewing directions(AVG) (either as absolute value or as a percentage of the range from thesmallest intersection angle to the largest intersection angle in onepermutation) and the standard deviation (SD) associated therewith foreach permutation of irradiation unit positions to be analysed. Thefollowing pseudocode example uses an average intersection angle ofgreater than or equal to 60° in absolute value or an averageintersection angle of greater than or equal to 70% the range in arelative definition, each at an associated standard deviation of lessthan or equal to 10°:

currentSequenceOfBeams={(TreatmentBeamAngle1, TreatmentBeamAngle2, . . ., TreatmentBeamAngleN};

Range=Max(currentSequenceOfBeams)−Min(currentSequenceOfBeams); //Compute Range between maximum and minimum intersection angle

int n=size(currentSequenceOfBeams);

int i=0;

Termination=false;

Repeat:

-   -   i++;    -   computeAVGandSD(currentSequenceOfBeams)=>AVG and SD; // Compute        average (e.g. the arithmetic mean value) of acute intersection        angles and the associated standard deviation for        currentSequenceOfBeams    -   If ((AVG>=60°) && (SD<=10°))        -   Termination=true;    -   Else If ((AVG>=70%*Range) && (SD <=10°))        -   Termination=true;    -   Else computeNextPermutation(currentSequenceOfBeams); // this may        result in a change to currentSequenceOfBeams like={beam2, beam1,        . . . , beamN}

Until (i>=n! ∥ Termination==true) // acceptance criterion forcurrentSequenceOfBeams

The meaning of the variables used in the above pseudocode is as follows:

currentSequenceOfBeams: represents an input (reading) of the unorderedsequence of beam directions/irradiation unit positions and/orintersection angles TreatmentBeamAnglei (i=1, 2, . . . , N);

Range: angular value representing the difference between the largestvalue in currentSequenceOfBeams to the smallest value incurrentSequenceOfBeams.

The meaning of operators in the above pseudocode is as follows:

>=: greater than or equal to;

<=less than or equal to;

*: multiplication;

=: set to;

&&: logical “AND”;

==: identical to (for Boolean variables);

!: factorial;

∥: logical “OR”.

// indicates the beginning of a comment on a code line.

If the imaging device comprises at least two imaging units, thedisclosed method may comprise determining, based on the irradiation unitposition data, stereo-imaging data describing (for example, defining)irradiation unit positions for which all of the at least two imagingunits have a free viewing direction. The optimal order data may then bedetermined based on the stereo-imaging data, for example by preferringorders described by the position order data having a maximum ofirradiation unit positions for which all of the at least two imagingunits have a free viewing direction for determining the optimal orderdata. Also, imaging unit control data may be determined based on theirradiation unit position data. The imaging unit control data describes(for example defines) a control signal for controlling only the at leastone of the at least two imaging units which has a free viewing directionto take an image.

In a second aspect, the disclosed method relates to acomputer-implemented medical data processing method for determining apositional pattern of an irradiation unit for irradiating a patient withtreatment radiation, the irradiation unit being part of an irradiationtreatment device having an imaging device having two imaging units forgenerating a medical image of the patient, the method comprisingexecuting, on at least one processor of at least one computer, steps of:

-   -   a) acquiring, at the at least one processor, irradiation unit        position data describing irradiation unit positions of the        irradiation unit for which the imaging device has a free viewing        direction onto the position of the patient which is a free line        of sight from at least one of the imaging units to the position        of the patient;    -   b) determining, by the at least one processor and based on the        irradiation unit position data, irradiation unit control data        describing control signals for controlling the irradiation unit        to alternately attain irradiation unit positions described by        the irradiation unit position data, wherein the free line of        sight of each a different one of the two imaging units is        impeded by at least part of the irradiation unit in two        immediately subsequently attained irradiation unit positions.

The terminology used above for defining the second aspect is defined andto be understood like the terminology used for describing the firstaspect.

In a third aspect, the disclosed method relates to controlling theposition of an irradiation unit of an irradiation treatment device, themethod comprising executing, on at least one processor of at least onecomputer, steps of:

-   -   executing the method according to any of the first or second        aspects; and    -   issuing, by the at least one processor and to the irradiation        treatment device and based on the optimal order data, control        signal data for controlling the imaging unit to attain the        imaging positions in the order described by the optimal order        data.

In a fourth aspect, the invention is directed to a computer programwhich, when running on at least one processor (for example, a processor)of at least one computer (for example, a computer) or when loaded intoat least one memory (for example, a memory) of at least one computer(for example, a computer), causes the at least one computer to performthe above-described method according to the first, second or thirdaspects.

In a fifth aspect, the invention is directed to a non-transitorycomputer-readable program storage medium on which the program accordingto the fourth aspect is stored.

In a sixth aspect, the invention is directed to at least one computer(for example, a computer), comprising at least one processor (forexample, a processor) and at least one memory (for example, a memory),wherein the program according to the fourth aspect is running on theprocessor or is loaded into the memory, or wherein the at least onecomputer comprises the program storage medium according to the fifthaspect.

In a seventh aspect, the invention is directed to a system fordetermining a positional pattern of an irradiation unit for irradiatinga patient with treatment radiation, the system comprising:

-   -   a) the at least one computer according to the sixth aspect; and    -   b) at least one electronic data storage device storing the        irradiation unit position data and, as far as the irradiation        unit position data is predetermined and has been generated on        the basis of treatment plan data, the treatment plan data,    -   wherein the at least one computer is operably coupled to the at        least one electronic data storage device for acquiring, from the        at least one electronic data storage device, the irradiation        unit position data and, if the electronic data storage device        stores the treatment plan data, the treatment plan data.

In an eighth aspect, the invention is directed to a radiation treatmentsystem, comprising:

-   -   an irradiation treatment device comprising an irradiation unit        and an imaging device; and    -   the at least one computer according to the sixth aspect or the        system according to the seventh aspect,    -   wherein the at least one computer is operatively coupled to the        irradiation treatment device for controlling the position of at        least the irradiation unit on the basis of the optimal order        data.

It is within the scope of the present invention to combine one or morefeatures of one or more embodiments or aspects of the invention in orderto form a new embodiment wherever this is technically expedient and/orfeasible. Specifically, a feature of one embodiment which has the sameor a similar function to another feature of another embodiment can beexchanged with said other feature, and a feature of one embodiment whichadds an additional function to another embodiment can for example beadded to said other embodiment.

In this section, definitions for specific terminology used in thisdisclosure are offered which also form part of the present disclosure.

The method in accordance with the invention is for example a computerimplemented method. For example, all the steps or merely some of thesteps (i.e. less than the total number of steps) of the method inaccordance with the invention can be executed by a computer (forexample, at least one computer). An embodiment of the computerimplemented method is a use of the computer for performing a dataprocessing method. An embodiment of the computer implemented method is amethod concerning the operation of the computer such that the computeris operated to perform one, more or all steps of the method.

The computer for example comprises at least one processor and forexample at least one memory in order to (technically) process the data,for example electronically and/or optically. The processor being forexample made of a substance or composition which is a semiconductor, forexample at least partly n- and/or p-doped semiconductor, for example atleast one of II-, III-, IV-, V-, VI-semiconductor material, for example(doped) silicon and/or gallium arsenide. The calculating steps describedare for example performed by a computer. Determining steps orcalculating steps are for example steps of determining data within theframework of the technical method, for example within the framework of aprogram. A computer is for example any kind of data processing device,for example electronic data processing device. A computer can be adevice which is generally thought of as such, for example desktop PCs,notebooks, netbooks, etc., but can also be any programmable apparatus,such as for example a mobile phone or an embedded processor. A computercan for example comprise a system (network) of “sub-computers”, whereineach sub-computer represents a computer in its own right. The term“computer” includes a cloud computer, for example a cloud server. Theterm “cloud computer” includes a cloud computer system which for examplecomprises a system of at least one cloud computer and for example aplurality of operatively interconnected cloud computers such as a serverfarm. Such a cloud computer is preferably connected to a wide areanetwork such as the world wide web (WWW) and located in a so-calledcloud of computers which are all connected to the world wide web. Suchan infrastructure is used for “cloud computing”, which describescomputation, software, data access and storage services which do notrequire the end user to know the physical location and/or configurationof the computer delivering a specific service. For example, the term“cloud” is used in this respect as a metaphor for the Internet (worldwide web). For example, the cloud provides computing infrastructure as aservice (IaaS). The cloud computer can function as a virtual host for anoperating system and/or data processing application which is used toexecute the method of the invention. The cloud computer is for examplean elastic compute cloud (EC2) as provided by Amazon Web Services™. Acomputer for example comprises interfaces in order to receive or outputdata and/or perform an analogue-to-digital conversion. The data are forexample data which represent physical properties and/or which aregenerated from technical signals. The technical signals are for examplegenerated by means of (technical) detection devices (such as for exampledevices for detecting marker devices) and/or (technical) analyticaldevices (such as for example devices for performing imaging methods),wherein the technical signals are for example electrical or opticalsignals. The technical signals for example represent the data receivedor outputted by the computer. The computer is preferably operativelycoupled to a display device which allows information outputted by thecomputer to be displayed, for example to a user. One example of adisplay device is an augmented reality device (also referred to asaugmented reality glasses) which can be used as “goggles” fornavigating. A specific example of such augmented reality glasses isGoogle Glass (a trademark of Google, Inc.). An augmented reality devicecan be used both to input information into the computer by userinteraction and to display information outputted by the computer.Another example of a display device would be a standard computer monitorcomprising for example a liquid crystal display operatively coupled tothe computer for receiving display control data from the computer forgenerating signals used to display image information content on thedisplay device. A specific embodiment of such a computer monitor is adigital lightbox. The monitor may also be the monitor of a portable, forexample handheld, device such as a smart phone or personal digitalassistant or digital media player.

The expression “acquiring data” for example encompasses (within theframework of a computer implemented method) the scenario in which thedata are determined by the computer implemented method or program.Determining data for example encompasses measuring physical quantitiesand transforming the measured values into data, for example digitaldata, and/or computing the data by means of a computer and for examplewithin the framework of the method in accordance with the invention. Themeaning of “acquiring data” also for example encompasses the scenario inwhich the data are received or retrieved by the computer implementedmethod or program, for example from another program, a previous methodstep or a data storage medium, for example for further processing by thecomputer implemented method or program. The expression “acquiring data”can therefore also for example mean waiting to receive data and/orreceiving the data. The received data can for example be inputted via aninterface. The expression “acquiring data” can also mean that thecomputer implemented method or program performs steps in order to(actively) receive or retrieve the data from a data source, for instancea data storage medium (such as for example a ROM, RAM, database, harddrive, etc.), or via the interface (for instance, from another computeror a network). The data acquired by the disclosed method or device,respectively, may be acquired from a database located in a data storagedevice which is operably to a computer for data transfer between thedatabase and the computer, for example from the database to thecomputer. The computer acquires the data for use as an input for stepsof determining data. The determined data can be output again to the sameor another database to be stored for later use. The database or databaseused for implementing the disclosed method can be located on networkdata storage device or a network server (for example, a cloud datastorage device or a cloud server) or a local data storage device (suchas a mass storage device operably connected to at least one computerexecuting the disclosed method). The data can be made “ready for use” byperforming an additional step before the acquiring step. In accordancewith this additional step, the data are generated in order to beacquired. The data are for example detected or captured (for example byan analytical device). Alternatively or additionally, the data areinputted in accordance with the additional step, for instance viainterfaces. The data generated can for example be inputted (for instanceinto the computer). In accordance with the additional step (whichprecedes the acquiring step), the data can also be provided byperforming the additional step of storing the data in a data storagemedium (such as for example a ROM, RAM, CD and/or hard drive), such thatthey are ready for use within the framework of the method or program inaccordance with the invention. The step of “acquiring data” cantherefore also involve commanding a device to obtain and/or provide thedata to be acquired. In particular, the acquiring step does not involvean invasive step which would represent a substantial physicalinterference with the body, requiring professional medical expertise tobe carried out and entailing a substantial health risk even when carriedout with the required professional care and expertise. In particular,the step of acquiring data, for example determining data, does notinvolve a surgical step and in particular does not involve a step oftreating a human or animal body using surgery or therapy. In order todistinguish the different data used by the present method, the data aredenoted (i.e. referred to) as “XY data” and the like and are defined interms of the information which they describe, which is then preferablyreferred to as “XY information” and the like.

Within the framework of the invention, computer program elements can beembodied by hardware and/or software (this includes firmware, residentsoftware, micro-code, etc.). Within the framework of the invention,computer program elements can take the form of a computer programproduct which can be embodied by a computer-usable, for examplecomputer-readable data storage medium comprising computer-usable, forexample computer-readable program instructions, “code” or a “computerprogram” embodied in said data storage medium for use on or inconnection with the instruction-executing system. Such a system can be acomputer; a computer can be a data processing device comprising meansfor executing the computer program elements and/or the program inaccordance with the invention, for example a data processing devicecomprising a digital processor (central processing unit or CPU) whichexecutes the computer program elements, and optionally a volatile memory(for example a random access memory or RAM) for storing data used forand/or produced by executing the computer program elements. Within theframework of the present invention, a computer-usable, for examplecomputer-readable data storage medium can be any data storage mediumwhich can include, store, communicate, propagate or transport theprogram for use on or in connection with the instruction-executingsystem, apparatus or device. The computer-usable, for examplecomputer-readable data storage medium can for example be, but is notlimited to, an electronic, magnetic, optical, electromagnetic, infraredor semiconductor system, apparatus or device or a medium of propagationsuch as for example the Internet. The computer-usable orcomputer-readable data storage medium could even for example be paper oranother suitable medium onto which the program is printed, since theprogram could be electronically captured, for example by opticallyscanning the paper or other suitable medium, and then compiled,interpreted or otherwise processed in a suitable manner. The datastorage medium is preferably a non-volatile data storage medium. Thecomputer program product and any software and/or hardware described hereform the various means for performing the functions of the invention inthe example embodiments. The computer and/or data processing device canfor example include a guidance information device which includes meansfor outputting guidance information. The guidance information can beoutputted, for example to a user, visually by a visual indicating means(for example, a monitor and/or a lamp) and/or acoustically by anacoustic indicating means (for example, a loudspeaker and/or a digitalspeech output device) and/or tactilely by a tactile indicating means(for example, a vibrating element or a vibration element incorporatedinto an instrument). For the purpose of this document, a computer is atechnical computer which for example comprises technical, for exampletangible components, for example mechanical and/or electroniccomponents. Any device mentioned as such in this document is a technicaland for example tangible device.

The information on the imaging geometry preferably comprises informationwhich allows the analysis image (x-ray image) to be calculated, given aknown relative position between the imaging geometry analysis apparatusand the analysis object (anatomical body part) to be analysed by x-rayradiation, if the analysis object which is to be analysed is known,wherein “known” means that the spatial geometry (size and shape) of theanalysis object is known. This means for example that three-dimensional,“spatially resolved” information concerning the interaction between theanalysis object (anatomical body part) and the analysis radiation (x-rayradiation) is known, wherein “interaction” means for example that theanalysis radiation is blocked or partially or completely allowed to passby the analysis object. The location and in particular orientation ofthe imaging geometry is for example defined by the position of the x-raydevice, for example by the position of the x-ray source and the x-raydetector and/or for example by the position of the multiplicity(manifold) of x-ray beams which pass through the analysis object and aredetected by the x-ray detector. The imaging geometry for exampledescribes the position (i.e. the location and in particular theorientation) and the shape (for example, a conical shape exhibiting aspecific angle of inclination) of said multiplicity (manifold). Theposition can for example be represented by the position of an x-ray beamwhich passes through the centre of said multiplicity or by the positionof a geometric object (such as a truncated cone) which represents themultiplicity (manifold) of x-ray beams. Information concerning theabove-mentioned interaction is preferably known in three dimensions, forexample from a three-dimensional CT, and describes the interaction in aspatially resolved way for points and/or regions of the analysis object,for example for all of the points and/or regions of the analysis object.Knowledge of the imaging geometry for example allows the location of asource of the radiation (for example, an x-ray source) to be calculatedrelative to an image plane (for example, the plane of an x-raydetector). With respect to the connection between three-dimensionalanalysis objects and two-dimensional analysis images as defined by theimaging geometry, reference is made for example to the followingpublications:

Image fusion can be elastic image fusion or rigid image fusion. In thecase of rigid image fusion, the relative position between the pixels ofa 2D image and/or voxels of a 3D image is fixed, while in the case ofelastic image fusion, the relative positions are allowed to change.

In this application, the term “image morphing” is also used as analternative to the term “elastic image fusion”, but with the samemeaning.

Elastic fusion transformations (for example, elastic image fusiontransformations) are for example designed to enable a seamlesstransition from one dataset (for example a first dataset such as forexample a first image) to another dataset (for example a second datasetsuch as for example a second image). The transformation is for exampledesigned such that one of the first and second datasets (images) isdeformed, for example in such a way that corresponding structures (forexample, corresponding image elements) are arranged at the same positionas in the other of the first and second images. The deformed(transformed) image which is transformed from one of the first andsecond images is for example as similar as possible to the other of thefirst and second images. Preferably, (numerical) optimisation algorithmsare applied in order to find the transformation which results in anoptimum degree of similarity. The degree of similarity is preferablymeasured by way of a measure of similarity (also referred to in thefollowing as a “similarity measure”). The parameters of the optimisationalgorithm are for example vectors of a deformation field. These vectorsare determined by the optimisation algorithm in such a way as to resultin an optimum degree of similarity. Thus, the optimum degree ofsimilarity represents a condition, for example a constraint, for theoptimisation algorithm. The bases of the vectors lie for example atvoxel positions of one of the first and second images which is to betransformed, and the tips of the vectors lie at the corresponding voxelpositions in the transformed image. A plurality of these vectors ispreferably provided, for instance more than twenty or a hundred or athousand or ten thousand, etc. Preferably, there are (other) constraintson the transformation (deformation), for example in order to avoidpathological deformations (for instance, all the voxels being shifted tothe same position by the transformation). These constraints include forexample the constraint that the transformation is regular, which forexample means that a Jacobian determinant calculated from a matrix ofthe deformation field (for example, the vector field) is larger thanzero, and also the constraint that the transformed (deformed) image isnot self-intersecting and for example that the transformed (deformed)image does not comprise faults and/or ruptures. The constraints includefor example the constraint that if a regular grid is transformedsimultaneously with the image and in a corresponding manner, the grid isnot allowed to interfold at any of its locations. The optimising problemis for example solved iteratively, for example by means of anoptimisation algorithm which is for example a first-order optimisationalgorithm, such as a gradient descent algorithm. Other examples ofoptimisation algorithms include optimisation algorithms which do not usederivations, such as the downhill simplex algorithm, or algorithms whichuse higher-order derivatives such as Newton-like algorithms. Theoptimisation algorithm preferably performs a local optimisation. Ifthere is a plurality of local optima, global algorithms such assimulated annealing or generic algorithms can be used. In the case oflinear optimisation problems, the simplex method can for instance beused.

In the steps of the optimisation algorithms, the voxels are for exampleshifted by a magnitude in a direction such that the degree of similarityis increased. This magnitude is preferably less than a predefined limit,for instance less than one tenth or one hundredth or one thousandth ofthe diameter of the image, and for example about equal to or less thanthe distance between neighbouring voxels. Large deformations can beimplemented, for example due to a high number of (iteration) steps.

The determined elastic fusion transformation can for example be used todetermine a degree of similarity (or similarity measure, see above)between the first and second datasets (first and second images). To thisend, the deviation between the elastic fusion transformation and anidentity transformation is determined. The degree of deviation can forinstance be calculated by determining the difference between thedeterminant of the elastic fusion transformation and the identitytransformation. The higher the deviation, the lower the similarity,hence the degree of deviation can be used to determine a measure ofsimilarity.

A measure of similarity can for example be determined on the basis of adetermined correlation between the first and second datasets.

The present invention relates to the field of controlling a treatmentbeam. The treatment beam treats body parts which are to be treated andwhich are referred to in the following as “treatment body parts”. Thesebody parts are for example parts of a patient's body, i.e. anatomicalbody parts.

The present invention relates to the field of medicine and for exampleto the use of beams, such as radiation beams, to treat parts of apatient's body, which are therefore also referred to as treatment beams.A treatment beam treats body parts which are to be treated and which arereferred to in the following as “treatment body parts”. These body partsare for example parts of a patient's body, i.e. anatomical body parts.Ionising radiation is for example used for the purpose of treatment. Forexample, the treatment beam comprises or consists of ionising radiation.The ionising radiation comprises or consists of particles (for example,sub-atomic particles or ions) or electromagnetic waves which areenergetic enough to detach electrons from atoms or molecules and soionise them. Examples of such ionising radiation include x-rays,high-energy particles (high-energy particle beams) and/or ionisingradiation emitted from a radioactive element. The treatment radiation,for example the treatment beam, is for example used in radiation therapyor radiotherapy, such as in the field of oncology. For treating cancerin particular, parts of the body comprising a pathological structure ortissue such as a tumour are treated using ionising radiation. The tumouris then an example of a treatment body part.

The treatment beam is preferably controlled such that it passes throughthe treatment body part. However, the treatment beam can have a negativeeffect on body parts outside the treatment body part. These body partsare referred to here as “outside body parts”. Generally, a treatmentbeam has to pass through outside body parts in order to reach and sopass through the treatment body part.

Reference is also made in this respect to the following web pages:http://www.elekta.com/healthcare_us_elekta_vmat.php andhttp://www.varian.com/us/oncology/treatments/treatment_techniques/rapidarc.

A treatment body part can be treated by one or more treatment beamsissued from one or more directions at one or more times. The treatmentby means of the at least one treatment beam thus follows a particularspatial and temporal pattern. The term “beam arrangement” is then usedto cover the spatial and temporal features of the treatment by means ofthe at least one treatment beam. The beam arrangement is an arrangementof at least one treatment beam.

The “beam positions” describe the positions of the treatment beams ofthe beam arrangement. The arrangement of beam positions is referred toas the positional arrangement. A beam position is preferably defined bythe beam direction and additional information which allows a specificlocation, for example in three-dimensional space, to be assigned to thetreatment beam, for example information about its co-ordinates in adefined co-ordinate system. The specific location is a point, preferablya point on a straight line. This line is then referred to as a “beamline” and extends in the beam direction, for example along the centralaxis of the treatment beam. The defined co-ordinate system is preferablydefined relative to a treatment device (also called irradiationtreatment device) which emits the treatment beam(s) or relative to atleast a part of the patient's body. The positional arrangement comprisesand for example consists of at least one beam position, for example adiscrete set of beam positions (for example, two or more different beampositions), or a continuous multiplicity (manifold) of beam positions.

For example, one or more treatment beams adopt(s) the treatment beamposition(s) defined by the positional arrangement simultaneously orsequentially during treatment (for example sequentially if there is onlyone beam source to emit a treatment beam). If there are several beamsources, it is also possible for at least a subset of the beam positionsto be adopted simultaneously by treatment beams during the treatment.For example, one or more subsets of the treatment beams can adopt thebeam positions of the positional arrangement in accordance with apredefined sequence. A subset of treatment beams comprises one or moretreatment beams. The complete set of treatment beams which comprises oneor more treatment beams which adopt(s) all the beam positions defined bythe positional arrangement is then the beam arrangement.

In the field of medicine, imaging methods (also called imagingmodalities and/or medical imaging modalities) are used to generate imagedata (for example, two-dimensional or three-dimensional image data) ofanatomical structures (such as soft tissues, bones, organs, etc.) of thehuman body. The term “medical imaging methods” is understood to mean(advantageously apparatus-based) imaging methods (so-called medicalimaging modalities and/or radiological imaging methods) such as forinstance computed tomography (CT) and cone beam computed tomography(CBCT, such as volumetric CBCT), x-ray tomography, magnetic resonancetomography (MRT or MRI), conventional x-ray, sonography and/orultrasound examinations, and positron emission tomography. The imagedata thus generated is also termed “medical imaging data”. Analyticaldevices for example are used to generate the image data inapparatus-based imaging methods. The imaging methods are for exampleused for medical diagnostics, to analyse the anatomical body in order togenerate images which are described by the image data. The imagingmethods are also for example used to detect pathological changes in thehuman body. However, some of the changes in the anatomical structure,such as the pathological changes in the structures (tissue), may not bedetectable and for example may not be visible in the images generated bythe imaging methods. A tumour represents an example of a change in ananatomical structure. If the tumour grows, it may then be said torepresent an expanded anatomical structure. This expanded anatomicalstructure may not be detectable; for example, only a part of theexpanded anatomical structure may be detectable. Primary/high-gradebrain tumours are for example usually visible on MRI scans when contrastagents are used to infiltrate the tumour. MRI scans represent an exampleof an imaging method. In the case of MRI scans of such brain tumours,the signal enhancement in the MRI images (due to the contrast agentsinfiltrating the tumour) is considered to represent the solid tumourmass. Thus, the tumour is detectable and for example discernible in theimage generated by the imaging method. In addition to these tumours,referred to as “enhancing” tumours, it is thought that approximately 10%of brain tumours are not discernible on a scan and are for example notvisible to a user looking at the images generated by the imaging method.

FIGS. 1a to 1d illustrate a sequence of gantry positions (indicated bythe amount of degrees resembling the rotational phase of the gantryshown above each sub-figure) of a gantry 4 of an irradiation treatmentdevice 3 having a couch 5 for placing a patient 8 onto. Lines 6 and 7indicate the viewing directions from x-ray emitters 2, 2′ to flat panelx-ray detectors 1, 1′ of a stereo x-ray camera being the imaging device(each pair of emitters 2, 2′ and detectors 1, 1′ forming each oneimaging unit). As can be seen in FIGS. 1a to 1 d, the viewing directionof one of the imaging unit is always blocked by the gantry 4 in each ofthe positions shown. Thus, subsequent imaging positions do not allowreconstructing stereo-view images from a series of images taken when thegantry is in the positions (irradiation unit positions) shown in FIGS.1a to 1 d. The order of irradiation unit positions shown in FIGS. 1a to1d is designed to minimize the travel time of the gantry 4 when goingthrough that ordered sequence of irradiation unit positions.

FIGS. 1e to 1h show a re-sorting of the irradiation unit positions sothat each a different viewing direction between alternating pairs ofemitters 2, 2′and detectors 1, 1′ (i.e. the viewing direction of each adifferent one of the imaging unit for subsequent images of the sequence)is obscured, thus allowing reconstructing of stereo-view images frompairs of images taken for subsequent irradiation unit positions: forexample, one stereo image can be reconstructed from the individualimages taken in the irradiation unit positions at 20° and 110° shown inFIGS. 1e and 1 f, one stereo image can be reconstructed from theindividual images taken in the irradiation unit positions at 110° and60° shown in FIGS. 1f and 1g , and one stereo image can be reconstructedfrom the individual images taken in the irradiation unit positions at60° an 130° shown in FIGS. 1g and 1 h. Throughout FIGS. 1 1 i and 1 j,the straight lines titled “resulting reconstruction angles” illustratethe intersecting viewing direction associated with subsequentirradiation unit positions of FIGS. 1a to 1d and 1e to 1 h,respectively.

Throughout FIG. 3, the circular diagram represents a mapping of thefield of view onto a section through patient's body, wherein the crossedshading indicates the combined field of view of the adjacent irradiationunit positions, and the linear shading indicates the spatial sectorwhich is not visible even after combination of the images taken at twoadjacent irradiation unit positions.

FIGS. 2a to 2l illustrate the difference between the disclosed methodand the state of the art for the case of an imaging device having only asingle imaging unit comprising one x-ray emitter unit 11 and one x-raydetector unit 22. Throughout FIGS. 2m, 2n and 2o , the straight linestitled “resulting reconstruction angles” illustrate the intersectingviewing direction associated with subsequent irradiation unit positionsof FIGS. 2a to 2d, 2e to 2h and 2i to 2 l, respectively. Compared to theprior art sequence of FIGS. 2a to 2d which again seeks to minimize thetravel time of the gantry 4, the disclosed method changes the order ofirradiation unit positions for taking images with the single imagingunit to follow the sequences shown in FIGS. 2e to 2h or 2 i to 2 l,respectively. The latter two sequences are determined such that anintersection angle between viewing directions 8 associated withimmediately subsequent (adjacent) irradiation unit positions (andtherefore in the case of FIG. 2 also subsequent positions of the viewingdirection relative to the patient place on the couch 5, and the combinedfield of view) is on average maximized and enlarged (see also therepresentation of intersecting viewing directions 8 between the adjacentfigures) compared to the prior art case of FIGS. 2a to 2d . Thissupports easier and more reliable fusion of subsequent images so thatthe reconstruction of stereo images is improved. Both ordered sequencesof FIGS. 2e to 2h and 2i to 2l have a higher average combined field ofview than the sequence of FIGS. 2a to 2d , but the sequence of FIGS. 2ito 2l is associated with a smaller standard deviation of theintersection angle and therefore selected as the optimal order of theirradiation unit positions, if the combined field of view FIGS. 2i to 2lis the same as the combined field of view of FIGS. 2e to 2 h.

FIG. 3 illustrates application of the disclosed method to the case ofthe imaging device being a stereo thermal camera having twoinfrared-sensitive imaging units 9, 9′. FIGS. 3e to 3h illustrate thatthe combined field of view (FoV) achievable by combining (e.g. fusing)subsequent images taken by the two imaging units 9, 9′ by optimizingtheir order is on average enlarged (crossed shading in the circle shownin FIG. 3j and surrounding the intersecting subsequent viewingdirections 6, 7) when applying the disclosed method (refer to FIGS. 3eto 3h and 3j ) compared to the prior art order of irradiation unitpositions (refer to FIGS. 3a to 3d and 3i ). The linear shading in thecircles of FIGS. 3i and 3j indicate the angular range which is obscuredand cannot be viewed and/or imaged for a combination of subsequentimaging positions sown in FIGS. 3a to 3d and 3e to 3h , respectively.Note that for application of a stereo thermal camera as shown in FIG. 3,the number of irradiation unit positions associated with an obscuredviewing direction 6, 7 is decreased by 50°% compared to the case ofFIGS. 1 and 2 in which x-ray-based imaging devices are used, because thepatient is imaged only from one hemisphere in three-dimensional space.

FIG. 4 shows the information derivable from fused stereo images taken bytwo different imaging units (Imager1 and Imager2). An example of suchinformation is knowledge that the patient is off the desired irradiationposition relative to the irradiation unit by a certain amount in e.g.three coordinates, or that the irradiation unit and the patient areperfectly aligned.

When having only one X-ray image of the scene for image positionverification, an in-plane two-dimensional fusion only may be performed.In doing so, no positional information is retrieved and it is onlychecked whether the in-plane shift is within a certain tolerance. Usingthe disclosed method, the latest image from the other—possiblyobstructed view—can be used in combination with the live X-ray image(non-obstructed) to perform a six-dimensional (three translationaldegrees of freedom, three rotational degrees of freedom) fusion. Visualinspection of this six-dimensional fusion result yields whether thestereo image pair was taken at the same position of the patient. If so,the following can be determined:

a) a quantified offset in 6D (six degrees of freedom, namely threetranslational degrees of freedom and three rotational degrees offreedom); and

b) the point in time the error was introduced.

If for instance the error was already introduced when using Imager1 butin the line of sight of Imager1, it can be determined when this errorwas introduced by combining the just acquired X-ray from Imager2 withthe latest images from Imager1. The image pair that is not combinable(because it showed a different position of the patient) determines thepoint in time. The thereby gained information can answer questions like:Shall I continue treatment? What do I have to take into account for thenext treatment plan (OARs, Target doses etc.)?

An example algorithm for the disclosed method when having non-staticcamera setup is:

1) Make a list of all viewing directions that have clear line of sight

2) Generate all possible combinations/orderings for these viewingdirections

3) Compute for all orderings the sum of the (acute-angled) intersectionangles of two consecutive viewing angles

4) Determine which ordering provides the maximum of this summation

5) In case there is more than one solution, take the one providing thesmaller standard deviation

6) In case there is still more than one solution, take the solution thatis easier for the operator (reduces gantry travelling time)

FIG. 5 is a flow diagram illustrating the basic steps of the disclosedmethod which in the illustrative example of FIG. 5 starts with a step S1of acquiring irradiation unit position data, corresponding todetermining gantry angles in FIGS. 1, 2 and 3 which are associated witha free line of sight. In subsequent step S2, the position orders data isdetermined so as to achieve an ordering of the gantry positions as shownin FIGS. 1e to 1h and 3e to 3h , respectively, and 2 e to 2 h or 2 i to2 l. This is followed by step S3 in which the intersection angle data isdetermined to assess e.g. the average field of view and—if necessary toachieve a unique result—the standard deviation associated with the fieldof view for subsequent irradiation unit positions. In step S4, theoptimal order data is then determined as described above.

1. A method for determining a positional pattern of an irradiation unitfor irradiating a patient with treatment radiation, the irradiation unitbeing part of an irradiation treatment system including at least oneprocessor and an irradiation treatment device having an imaging devicefor generating a medical image of the patient, the method comprising: a)acquiring, at the at least one processor, irradiation unit position datadescribing irradiation unit positions of the irradiation unit for whichthe imaging device has a free viewing direction onto the position of thepatient which is a free line of sight from the imaging device to theposition of the patient; b) determining, by the at least one processorand based on the irradiation unit position data, position orders datadescribing all possible orders of the irradiation unit positions forwhich the imaging device has a free viewing direction onto the positionof the patient; c) determining, by the at least one processor and basedon the irradiation unit position data and the position orders data andper order described by the position orders data, intersection angle datadescribing a statistical quantity of the intersection angles betweenfree viewing directions of the imaging unit for irradiation unitpositions which are immediately subsequent in the order described by theposition orders data; d) determining, by the at least one processor andbased on the intersection angle data and the position orders data,optimal order data describing an order of the irradiation unit positionsfor which the statistical value is optimal.
 2. The method according toclaim 1, wherein the irradiation unit position data is predetermined andhas been generated on the basis of treatment plan data describing anirradiation treatment plan defining treatment positions which arerelative positions between the irradiation treatment device and atreatment body part of the patient at which the treatment radiationshall be emitted.
 3. The method according to claim 1, wherein theimaging device comprises two imaging units.
 4. The method according toclaim 3, wherein the two imaging units each are one of an x-ray device,an infrared-sensitive imaging device, or a range camera device.
 5. Themethod according to claim 1, wherein the free line of sight is notimpeded by a part of the irradiation unit.
 6. The method according toclaim 1, wherein the irradiation unit position data is predetermined andhas been generated on the basis of device geometry data describing anoperational geometry of the irradiation treatment device.
 7. The methodaccording to claim 1, wherein the statistical quantity is a sum of theintersection angles, the average of the intersection angles, or theaverage field of view of the imaging unit between the viewing directionsassociated with the intersection angles.
 8. The method according toclaim 1, wherein the optimal order is an order for which the statisticalquantity is a maximum.
 9. The method according to claim 1, wherein, ifmore than one order of the irradiation unit position is associated withan optimum of the statistical quantity, the order of the irradiationunit positions associated with the smallest standard deviation isdetermined as the optimal order.
 10. The method according to claim 9,wherein, if more than one order of the irradiation unit position isassociated with a smallest standard deviation, the order of theirradiation unit positions associated with the smallest amount of timefor moving the irradiation unit through the order of irradiation unitpositions is determined as the optimal order.
 11. The method accordingto claim 1, wherein the intersection angles consist of obtuse angles andright angles or wherein the intersection angles consist of acute anglesand right angles.
 12. The method according to claim 1, wherein theintersection angles define a field of view of the imaging device betweenpairwise subsequent irradiation unit positions.
 13. The method accordingto claim 1, wherein the imaging device comprises at least two imagingunits, and wherein the irradiation unit positions are positions of theirradiation unit for which one of the at least two imaging units has afree viewing direction or wherein the irradiation unit positions arepositions of the irradiation unit for which each of the at least twoimaging units has a free viewing direction.
 14. The method according toclaim 13, comprising: determining, based on the irradiation unitposition data, stereo-imaging data describing irradiation unit positionsfor which all of the at least two imaging units have a free viewingdirection, and wherein the optimal order data is determined based on thestereo-imaging data.
 15. The method according claim 13, comprising:determining, based on the irradiation unit position data and by the atleast one processor, imaging unit control data describing a controlsignal for controlling only a one of the at least two imaging unitswhich has a free viewing direction to take an image.
 16. (canceled) 17.The method according to claim 1, comprising: issuing, by the at leastone processor and based on the optimal order data, control signal datafor controlling the imaging unit to attain the imaging positions in theorder described by the optimal order data; and controlling, by the atleast one processor, the imaging unit according to the control signaldata.
 18. (canceled)
 19. A non-transitory computer-readable storagemedium having instructions stored thereon, which, when executed by aprocessor of an irradiation treatment system including an irradiationtreatment device having an irradiation unit for irradiating a patientwith treatment radiation and an imaging device for generating a medicalimage of the patient, causes the processor to perform the stepscomprising: a) acquiring, at the at least one processor, irradiationunit position data describing irradiation unit positions of theirradiation unit for which the imaging device has a free viewingdirection onto the position of the patient which is a free line of sightfrom the imaging device to the position of the patient; b) determining,based on the irradiation unit position data, position orders datadescribing all possible orders of the irradiation unit positions forwhich the imaging device has a free viewing direction onto the positionof the patient; c) determining, and based on the irradiation unitposition data and the position orders data and per order described bythe position orders data, intersection angle data describing astatistical quantity of the intersection angles between free viewingdirections of the imaging unit for irradiation unit positions which areimmediately subsequent in the order described by the position ordersdata; and d) determining, based on the intersection angle data and theposition orders data, optimal order data describing an order of theirradiation unit positions for which the statistical value is optimal.20. (canceled)
 21. A system for determining a positional pattern of anirradiation unit for irradiating a patient with treatment radiation, thesystem comprising a processor and an irradiation treatment deviceincluding the irradiation unit for irradiating a patient with treatmentradiation and an imaging device for generating a medical image of thepatient, wherein the processor is configured to: a) acquire irradiationunit position data describing irradiation unit positions of theirradiation unit for which the imaging device has a free viewingdirection onto the position of the patient which is a free line of sightfrom the imaging device to the position of the patient; b) determine,based on the irradiation unit position data, position orders datadescribing all possible orders of the irradiation unit positions forwhich the imaging device has a free viewing direction onto the positionof the patient; c) determine, based on the irradiation unit positiondata and the position orders data and per order described by theposition orders data, intersection angle data describing a statisticalquantity of the intersection angles between free viewing directions ofthe imaging unit for irradiation unit positions which are immediatelysubsequent in the order described by the position orders data; and d)determine, based on the intersection angle data and the position ordersdata, optimal order data describing an order of the irradiation unitpositions for which the statistical value is optimal.
 22. (canceled) 23.The system of claim 21, wherein the processor is further configured to:issue, based on the optimal order data, control signal data forcontrolling the imaging unit to attain the imaging positions in theorder described by the optimal order data; and control the imaging unitaccording to the control signal data.
 24. The non-transitorycomputer-readable storage medium of claim 19, wherein the instructionscause the processor to perform the additional steps of: issuing, basedon the optimal order data, control signal data for controlling theimaging unit to attain the imaging positions in the order described bythe optimal order data; and controlling the imaging unit according tothe control signal data.